U.S. Pat. No. 3,521,280 discloses an information carrier of the general type to which the present invention relates and apparatus for reading it that operated at X-band frequencies. The carrier, in the form of a label, comprised a sheet of plastic overlain on one surface by a layer of electrically conductive material and on the other by small conducting strips that comprised resonant devices, each having a length equal to a quarter wave length of a predetermined frequency in the X-band. Such a carrier was suitable for situations where a large number of carriers bearing identical information were to be produced, but it was not suitable for inexpensive production where every carrier had to be encoded with information different from that on all of the others, or where only a few carriers at a time were encoded with identical information. Furthermore, the system provided for the encoding of only a relatively limited number of different items of information since the reader responded only to the presence or absence of predetermined frequencies without taking account of the sequence in which the resonant devices that were tuned to those frequencies were arranged across the face of the carrier.
U.S. Pat. No. 3,671,721 discloses a generally similar information carrier and reading apparatus with which it was used. The area of the carrier was divided into squares arranged in a pattern of columns and rows, and a resonant device was located in each of at least certain of the squares so that the information encoded on the carrier could be read by sensor means comprising a scanner. Each resonant device was in the form of a flat metallic spiral deposited on an insulating surface and comprising an inductive coil. The resonant devices were tuned to a limited number of different frequencies, and the information carrier was either produced by means of a mask depicting coils of the required frequencies or by making all coils initially of the same frequency and then entering data by removing parts or all of some of the coils to produce the required frequency distribution. Tuning such a resonant device by scraping away a portion of a coil was both troublesome and time consuming. Frequency adjustment by scraping tended to be somewhat inaccurate, and therefore it required that the resonant frequency of the adjusted device be checked after scraping. Of course the checking operation was likely to reveal the need for further adjustment, or even to reveal that the carrier had been spoiled by scraping away too much of a coil.
Swedish patent application No. 8404876-8 discloses a carrier generally like that last described but wherein each resonant device comprised a spiral coil of conductive material imprinted or otherwise deposited on one surface of an insulating sheet and a disc-like area of electrically conductive material on the opposite surface of that sheet that had about the same diameter as the coil and was substantially concentric with it. The disc-like deposit constituted a condenser plate that cooperated with the internal capacitance of the coil to provide the total capacitance of the device. As with the resonant device of U.S. Pat. No. 3,671,721, the resonant frequency of the device was retuned by scraping away a portion of the coil.